There are known various measuring techniques that use the Brillouin scattering phenomenon in an optical fiber (for example, Patent Document 1). One of the techniques is a distributed pressure sensor that utilizes a Brillouin frequency shift caused by strain applied to an optical fiber. Because the Brillouin frequency shift depends on strain applied to the optical fiber, pressure applied can be measured by analyzing the frequency shift of the optical fiber that is fixed to a material deformable by pressure.
The present inventors have already proposed a system for measuring mainly pressure and temperature distributions by mean of the Rayleigh frequency shift phenomenon as well as the Brillouin frequency shift phenomenon of an optical fiber (refer to Patent Document 2). Since this system aims to measure distributions of pressure and temperature and the optical fiber is not fixed to an object to be measured, strain measured by this system is of no use.
The pressure measurement technique using an optical fiber is applicable to volume change measurement of an object. For example, porous sandstone, because it changes in volume before and after containing liquid, is one application target of the above pressure measurement technique. Technologies for geologically sequestrating carbon dioxide are being developed recently as global warming countermeasures. In cases of implementing carbon dioxide geological sequestration, the foregoing pressure measurement technique can contribute to construction of systems for monitoring a sequestrated state of carbon dioxide in a sandstone and for monitoring geomechanical integrity and safety of a cap rock stratum (such as an argillaceous rock) that is the upper stratum on the sandstone.